
10
Circuit Protection Overview
Battery Pack
◦The battery pack has a thermistor device attached to the cells to monitor any over temperature
that may arise from fast charging/discharging or damage cells
Resettable Fuse
◦The battery pack is protected with a 3.75A polyfuse.
Thermal
◦The power switching transistors that control the charge and discharge is monitored by a thermistor.
Over-Current
◦The charge and discharge current is limited to 3A using a current sense resistor.
Battery Balance
◦While the circuit detects an input supply the cells are permanently monitored and kept balanced.
Reverse Polarity
◦Input is protected against reverse polarity and will apply a short to the input to blow the fuse in
the PSU unit.
Regenerative Power
◦Power regenerated by the doors motors being manually operated, is absorbed.
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PSU Input
The PSU Input is supplied from a separate unit which has a toroidal transformer, rectifier, reservoir capacitor
and a two fuses, one glass 20mm fuse on the mains supply and a poly fuse on the low voltage side. The
supply can deliver 3A at 40V.
The circuit has two load resistors R32&R33 which help with the mains fail detect speed response due to the
reservoir capacitor on the PSU being so large.
Diode D3 protects from reverse polarity protection.
PSU Output
The output is supplied either by the input via D1&D2 or from the battery via F1,R31,TR4,TR5.
Isolation
The Output is separated from the input by D1 & D2, which is required to allow for mains fail detection.
R1 reduces ghosting voltages from the result of switching the ground supply.
Charger Control
The charge is controlled by TR5 which in turn is controlled by the balancer, which is then again controlled by
the processor. If charge is allowed then TR5 is switched on and current is allowed to flow through the body
Diode of TR4